The present invention relates generally to the field of semiconductor plasma processing systems such as photoresist ashers, and more specifically to a gas distribution plate assembly for providing laminar gas flow across the surface of a substrate.
In the manufacture of integrated circuits, photolithography techniques are used to form integrated circuit patterns on a substrate, such a silicon wafer. Typically, the substrate is coated with a photoresist, portions of which are exposed to ultraviolet (UV) radiation through a mask to image a desired circuit pattern on the photoresist. The portions of the photoresist left unexposed to the UV radiation are removed by a processing solution, leaving only the exposed portions on the substrate. These remaining exposed portions are baked during a photostabilization process to enable the photoresist to withstand subsequent processing.
After such processing, in which the integrated circuit components are formed, it is generally necessary to remove the baked photoresist from the wafer. In addition, residue that has been introduced on the substrate surface through processes such as etching must be removed. Typically, the photoresist is xe2x80x9cashedxe2x80x9d or xe2x80x9cburnedxe2x80x9d and the ashed or burned photoresist, along with the residue, is xe2x80x9cstrippedxe2x80x9d or xe2x80x9ccleanedxe2x80x9d from the surface of the substrate.
One manner of removing photoresist and residues is by rapidly heating the photoresist-covered substrate in a vacuum chamber to a preset temperature by infrared radiation, and directing microwave-energized reactive gases (i.e., a plasma) toward the heated substrate surface. In the resulting process, the reactive plasma reacts with the photoresist to ash it for subsequent removal from the wafer.
It is important that the ashing process occur at substantially the same rate across the surface of the wafer. To insure such uniform ashing of the photoresist, the process conditions must be precisely controlled. Process conditions that must be so controlled include the temperature of the process chamber and the temperature of the wafer.
Known gas distribution or baffle plates for directing energized plasma toward a wafer are typically made of quartz, due to their ability to withstand high process temperatures. However, the use of quartz makes acceptable wafer and process temperature uniformity difficult to obtain. The temperature non-uniformities are caused by the large temperature gradients that can develop across the surface of a quartz plate due to its poor thermal conductivity characteristics. In addition, undesirable infrared (IR) wavelength absorption characteristics of quartz add to the thermal energy absorbed by the baffle plate. As a result, process uniformity and system throughput are adversely affected.
In addition to precise temperature control, the energized plasma that reacts with the photoresist must be evenly distributed across the wafer while a constant gas flow rate is maintained. Known baffle plates such as that shown U.S. Pat. No. 5,449,410 to Chang et al. distribute energized gases into the process chamber by means of a configuration that includes perimetric apertures but no apertures near the center (see FIG. 2 of Chang). However, known baffle plates such as that shown suffer from an inability to evenly distribute gas across the surface of the wafer in a laminar flow-like manner, especially when high gas flow rates are provided to achieve corresponding high process rates.
Thus, it is an object of the present invention to provide a mechanism for enabling a laminar flow of energized gas across the surface of a substrate being processed in a plasma processing system. The flow of gas is such that reactive species are supplied to the surface of the wafer in a manner that provides a uniform reaction rate even at high gas flow rates. This is achieved by providing a ratio of laminar jet center velocity to jet expansion such that a uniform mass flow rate per unit area is provided to the surface of the wafer. In addition, the supply of reactive species to the surface of the wafer allows for the generation of reaction effluent emanating from the surface of the wafer as the reaction occurs. It is a further object of the invention to improve wafer-to-wafer process uniformity in such a system. It is still a further object of the invention to provide a mechanism for minimizing temperature gradients across the wafer by providing a relatively flat temperature profile, across the surface of a gas distribution or baffle plate in such a system.
A baffle plate assembly is provided for distributing gas flow into an adjacent process chamber containing a semiconductor wafer to be processed. The baffle plate assembly comprises a generally planar upper baffle plate fixedly positioned above a generally planar lower baffle plate. A plenum is formed between the lower baffle plate and the process chamber lid or top wall. The lower baffle plate is sealed to the process chamber, and the process chamber top wall is attached to the lower baffle plate, creating a region of higher pressure in this plenum (as compared to the process chamber pressure). At least the lower baffle plate has a pattern of apertures formed therein for permitting gas to pass therethrough and into the wafer process chamber. The upper baffle plate and the lower baffle plate are positioned generally parallel to each other, and the upper baffle plate is smaller than the lower baffle plate. Preferably, the lower baffle plate is comprised of low-alloy anodized aluminum, and the upper baffle plate is comprised of sapphire-coated quartz.
In a 300 millimeter (mm) embodiment, the apertures in the lower baffle plate are arranged in a pattern such that each aperture is equidistant from any adjacent aperture. The upper baffle plate is provided with slightly larger apertures formed therein in a concentric multiply circular (radial) pattern. The upper baffle plate is also provided with a centrally located impingement plate. In a 200 mm embodiment, the apertures in the lower baffle plate arranged in a concentric multiply circular (radial) pattern, and the upper baffle plate is apertureless.